A feature of the present invention is to provide a vehicle brake of the initially described type, which is comfortably actuable to activate the parking braking function and with which higher clamping forces may be summoned up to guarantee a reliable parking braking function.
This feature is achieved by a vehicle brake of the initially described type, in which the actuating device comprises a motor drive, which is drive-connected to the ramp arrangement in such a way that the blocking element upon activation of the motor drive is displaced to secure and release the brake piston inside the housing.
According to the invention, the vehicle brake is accordingly equipped with a motor-driven actuating device, which makes it superfluous for the driver to summon up in an uncomfortable and purely mechanical manner the clamping force needed to activate the parking braking function. Rather, an activation of the parking braking function may be effected electronically, i.e. by pressing a button or through automatic recognition of an operating situation. The vehicle brake according to the invention moreover allows the actual displacement of the brake piston plus its brake lining to be effected hydraulically also upon activation of the parking braking function and—where necessary—allows the actuating device to cause merely a clamping force increase in addition to the hydraulically generated clamping force. The blocking device additionally has the function of securing the brake piston in its axial position achieved as a result of the hydraulic displacement, thereby making it possible to maintain a sufficiently high clamping force, which, even in the event of elasticity-induced and thermally induced dimensional changes of the components involved in the clamping path, guarantees a reliable parking braking function.
With regard to the motor drive, it may be provided that it takes the form of a rotary drive, for example a conventional rotary electric motor may be used. Proceeding from this development of the invention, it may be provided that the actuating device comprises an eccentric arrangement, by means of which the motor drive is connected to the ramp arrangement. In said case, the eccentric arrangement may comprise an eccentric disc connected to the motor drive as well as a swivelling lever connected to the ramp arrangement, wherein the swivelling lever upon activation of the motor drive slides on the eccentric disc and in so doing swivels. By suitably selecting the shape of the eccentric disc and of the external contour of the swivelling lever, a desired clamping force characteristic may be achieved. In a development of the invention, it is possible for the external contour of the eccentric disc and of the swivelling lever to have contour portions, which in a state of mutual abutment lock eccentric disc and swivelling lever relative to one another, thereby preventing unintentional cancellation of the parking braking function.
Instead of a rotary drive, the motor drive may take the form of a linear drive. This may be achieved for example by connecting a rotary drive to a threaded mechanism that converts the rotational movement to a linear movement. Given the use of a linear drive, a development of the invention provides that the actuating device comprises a lever arrangement, by means of which the motor drive is connected to the ramp arrangement.
The motor drive may comprise an electric motor. It is equally possible for the motor drive—when in the form of a linear drive—to comprise a solenoid. The construction of the motor drive in the form of a hydraulic motor, particularly in the form of a hydraulically controllable piston/cylinder arrangement, is also possible within the framework of the invention.
With regard to the ramp arrangement, it may be provided that it comprises at least one first ramp surface, which is provided on the housing or connected thereto in a rotationally fixed manner, and at least one second ramp surface formed on a ramp component, which is displaceable relative to the housing and connected to the blocking element, wherein upon activation of the motor drive the two ramp surfaces slide on one another to displace the blocking element. In a development of this idea, the invention provides that between the at least one first ramp surface and the at least one second ramp surface rolling bodies are provided, via which the at least one first ramp surface is in contact with the at least one second ramp surface. Upon activation of the motor drive, such a ramp arrangement behaves in such a way that the second ramp surface, which is displaceable relative to the housing, executes an axial movement relative to the housing. This axial movement may be used to displace the brake piston in the direction of its piston longitudinal axis and hence load it towards a brake disc. Preferably, a plurality of complementary ramp surfaces interact, which are formed in a complementary manner on the housing and on the ramp component respectively. Suitable rolling bodies are, for example, balls or rollers.
With regard to operation of the vehicle brake according to the invention, it is provided that in a service braking situation the brake piston is displaceable inside the housing by charging the fluid chamber with hydraulic fluid and discharging hydraulic fluid from the fluid chamber and that in a parking braking situation first the brake piston is displaced inside the housing by charging the fluid chamber with hydraulic fluid, then the actuating device is actuated to secure the brake piston, wherein the blocking element is fed towards the brake piston, and finally for a pressure reduction in the fluid chamber hydraulic fluid is discharged from the fluid chamber. It may further be provided that to cancel the parking braking situation the fluid chamber is charged with hydraulic fluid until the blocking element is released by the brake piston, then the blocking element is removed from the brake piston by means of the blocking device and finally for a pressure reduction in the fluid chamber hydraulic fluid is discharged from the fluid chamber. Thus, to activate the parking braking function it is possible to use the clamping force summoned up by a hydraulic actuation and displacement of the brake piston for placing and clamping the brake lining onto the brake disc. In addition, the clamping force summoned up by the actuating device may be additively superimposed on the hydraulically summoned-up clamping force. In this way, progressively and without a significant expenditure of force having to be summoned up mechanically by the driver, a higher clamping force is achieved for the parking braking situation than was described previously with regard to the background art according to EP 0 403 635 B1. The hydraulic pressure in the fluid chamber is then reduced to zero. In said case, the brake piston moves back slightly in the direction of its initial position.
With regard to the arising axial forces, in a development of the invention it is provided that in terms of magnitude the axial force component, which is mechanically exerted on the brake piston in addition to the hydraulically generated axial force component upon actuation of the actuating device, is greater than or equal to a minimum axial force, by which the axial force exerted by the brake piston on a component to be braked is reduced after the brake piston has been secured and the hydraulic fluid discharged from the fluid chamber. The clamping force is therefore, after the discharge of hydraulic fluid from the fluid chamber, still high enough to be able to guarantee a reliable parking braking effect. Even in the event of thermally induced dimensional changes, such as may occur for example when the individual vehicle brake components involved in the clamping path cool down at the end of an extended period of travel, or in the event of elastic deformations under load and subsequent relaxation, the clamping force summoned up according to the invention is still high enough to be able to guarantee a reliable parking braking effect.
To cancel the parking braking situation, in the previously described variant of the invention it is necessary to charge the fluid chamber anew with hydraulic fluid and hence cancel a mechanical connection between brake piston and blocking device. The actuating device may then be correspondingly controlled and the blocking element may move back into its initial position, in which it releases the brake piston. Hydraulic fluid may then be discharged from the fluid chamber, so that the brake piston may move back into a braking-effect-free initial position. As an alternative to such a procedure, with a motor drive of a sufficiently powerful design it is also possible, without a renewed feed of hydraulic fluid, to move the blocking element back into its initial position and accordingly release the brake piston, so that the brake piston may adopt its braking-effect-free position.
The invention further relates to a method of actuating a vehicle brake of the previously described type, wherein in a service braking situation the brake piston is displaced inside the housing by feeding hydraulic fluid to the fluid chamber or discharging hydraulic fluid from the fluid chamber, and that in a parking braking situation first the brake piston is displaced inside the housing by charging the fluid chamber with hydraulic fluid, then the actuating device is actuated to secure the brake piston, wherein the blocking element is fed towards the brake piston and presses thereon, and finally for a pressure reduction in the fluid chamber hydraulic fluid is discharged from the fluid chamber. In said case, it may be provided that the axial force component, which is mechanically exerted on the brake piston in addition to the hydraulically generated axial force component upon actuation of the actuating device, is selected in such a way that in terms of magnitude it is greater than or equal to a minimum axial force, by which the axial force exerted by the brake piston on a component to be braked is reduced after the brake piston has been secured and the hydraulic fluid discharged from the fluid chamber. The minimum axial force may be determined, for example, from the difference of the moduli of elasticity of the components involved in the hydraulic displacement of the brake piston and the components involved in the mechanical actuation of the actuating device.
To cancel the parking braking situation, the fluid chamber is charged with hydraulic fluid until the blocking element is at least partially relieved or released by the brake piston, then the blocking element is removed from the brake piston by means of the blocking device and finally for a pressure reduction in the fluid chamber hydraulic fluid is discharged from the fluid chamber.
Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the invention, when read in light of the accompanying drawings.